Electrical equipment and devices, and in particular high voltage equipment in an electrical power distribution system, have high heat dissipation and therefore require adequate cooling. For example, a conventional HVDC (High-Voltage Direct Current) converter valve may be air insulated and water-cooled. A cooling system is conventionally provided comprising for example cooling water distribution pipes that are shaped to fulfill certain requirements. Another example of an external cooling system is the use of fans.
However, there are also electrical devices within a power distribution system that are not cooled by any external cooling system. Those devices, lacking an external cooling system, are then instead only self-cooled, i.e. natural convective air-cooling. One example of such a self-cooled device is high voltage bushings, for example a converter transformer bushing.
Typical voltage levels within electrical power distribution systems range up to about 500 kV DC. However, the voltage levels increases constantly and may amount to as much as 800 kV DC and presumably even higher voltage levels in the future. Also, current levels may be up to 4000-5000 A or even higher. Naturally, such high voltages and current levels result in still higher heat dissipation and the requirements on electrical insulation of the bushing become extremely high. The size of the electrical insulation limits the cooling efficiency of the bushing, since the heat has to be led a longer distance to the ambient cooling air due to its increased size. The self-cooling is thus rendered insufficient at the very high voltage and current levels.
It would be feasible to utilize larger conductors when increasing the voltage levels, thereby lowering the heat dissipated, but this would again entail enlarging the equipment. That is, the size of the insulation would still be large.
Patent publication U.S. Pat. No. 2,953,629 is directed towards preventing flashovers in a condenser bushing, but also describes an attempt to cool bushings by means of a forced cooling mechanism. The cooling mechanism consists in sealing a fluid, such as water, within a bore of a central conductor. When the condenser bushing becomes heated, the liquid boils and vapor rises up and condense, whereupon the condensate returns to the bottom of the conductor. Heat is then transferred from the interior of the bushing through heat exchange tubes to the atmosphere.
The above-described prior art cooling arrangement entails a number of drawbacks. For example, the boiling point of the fluid defines the cooling temperature, which means that, in case the fluid is water, the cooling temperature is restricted to 100° C. It would be feasible to change the cooling temperature by altering the pressure, but this entails arranging pressure vessels, which would make the cooling mechanism cumbersome and expensive. In particular, such solution would involve a number of devices requiring high initial costs as well as having high maintenance costs. Another disadvantage is the risk of deposits on the equipment due to the vaporizing of water.
In view of the above, it would be desirable to enable improved cooling of high voltage devices, and in particular cooling of high voltage bushings. Further, it would also be desirable to provide a corresponding method for cooling such bushings.